As noted by [1]} in GX339-4, the X-ray emission in the dim
low/hard state can extrapolate back quite accurately to fit the
optical spectrum (see also [2]}; [3]}). One
way to produce this is via a single synchrotron component from the
innermost post-shock region of the jet. This is self absorbed in the
IR, strongly suppressing the emission at lower frequencies. As the
jet stretches out , each part of it produces a synchrotron component
which peaks at a lower frequency than those closer to the center. All
of these make the flat spectrum seen in the radio
(e.g. [4]}; [5]}). However, the non-thermal
synchrotron makes the optical and X-ray emission from a single
scattering in a single region, so it is hard to see how this can make
the weak anti-correlation between the optical and X-ray flux with lead
of a few hundred milliseconds which seems typical of this state
([1]}; [7]}; [8]}; [9]}).
